htmx 4.0: Fetch-based rewrite, built-in morphing swaps, explicit attribute inheritance

The htmx team released version 4.0, the first major release since 2024, skipping version 3. The library switched from XMLHttpRequest to fetch(), added built-in morphing swaps based on idiomorph, the hx-partial tag, and explicit attribute inheritance via the :inherited suffix. Script size remains about 14 KB, and version 2.x is supported indefinitely.

• Switch from XMLHttpRequest to fetch() enables native streaming at ~14 KB
• Morphing swaps based on idiomorph preserve DOM state, including input focus
• Attributes inherit only with :inherited suffix — otherwise CSRF token breaks
• Events renamed to htmx:phase:action pattern, added 60-second request timeout